Building Lean Systems Mixed Model Scheduling

Building Lean SystemsMixed Model Scheduling

2Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Mixed-Model Scheduling and Small Batch Production

In a perfect world, when the customer pulls a product from the final station, a signal is generated on each upstream resource to produce exactly what is pulled.

In the real world, changeover time, material availability, or operator availability motivates large batch production.

Large-lot production exacerbates Bowlwhip effect.

WIP between stations goes up, flow time goes up

3Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Mixed-Model Scheduling and Small Batch Production

Produce every product as quickly as possible, at the same rate at which customer demands are made.

 In a lean system, products flow smoothly through the enterprise with no delays

Mixed-model scheduling (heijunka) Evenly distributing the production of different

products over a period of time Changeover or setup times must be small Small batching in the presence of setup times

4Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Demand per hour A(3), B(2), and C(1). 10 min/part assembly time. 10 hrs/day, 5d/w.  Batch size is one week of demand; A(150),

B(100), C(50). Instead of receiving products every hour, the

customer will receive them once a week. The average inventory is 75 As, 50 Bs, and 25 Cs.

If production per hour was A(3), B(2), C(1), the finished goods inventory would be negligible because production would exactly match hourly demand.

MMS Example

5Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

AAABBC is a good hourly schedule, it could be more finely sequenced as follows: ABACAB.

Production in large batches uneven workload uneven demand for upstream processes, pull

impossible production is not synchronized with demand.

Mixed-model production: smooth work-load, smooth demand for upstream processes Allows production to match customer demand.

MMS Example

6Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Mixed Model Sequence for Volpens

Smallest possible sequence length = 1/0.09 = 11.11Choose a 11 unit sequence; adjust every two cycles.

Note: A mixed model sequence is used to intentionally vary work content and component requirements.

Model Monthly demand Ratio to Total # in the sequence 3 TimesBlue Pen 24 0.352941176 4.00 12Red Pen 14 0.205882353 2.33 7Yellow Pen 10 0.147058824 1.67 5Blue Shell 6 0.088235294 1.00 3Red Shell 8 0.117647059 1.33 4Yellow Shell 6 0.088235294 1.00 3

68 0.088235294 11.33333333 34Total Min

7Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Mixed Model Sequence For VolpensSeq. # Model Desc.1 Blue Pen2 Red Pen3 Blue Pen45

Yellow Pen67

Yellow Shell

8 Red Pen910

Yellow Pen

11

Blue Shell

121314151617

Seq. # Model Desc.181920212223242526272829303132

Red Shell3334

Blue Pen

Blue PenRed PenBlue Pen

Red Pen

Red ShellBlue Pen

Red PenYellow Pen

Blue Shell

Yellow ShellBlue PenRed PenBlue Pen

Yellow Pen

Red Shell

Red PenBlue Shell

Blue Pen

Yellow ShellBlue Pen

Red Shell

Blue Pen

Blue Pen

Yellow Pen

8Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Process 1 Process 2 Process 3Process 4

One Piece Flow

Reduce lead time by reducing WIPMove products between workstationsAverage lead time = WIP/ThroughputWhat is the average lead time for the

system above, if throughput is 100/week?2.4 weeks

9Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

One Piece FlowWhat is the average lead time for this system?

Process 1

1 unit

Process 2

1 unit

Process 3

1 unit

Process 4

1 unit

One piece could mean a pallet, a box etc. Is not practical in every situation One-piece flow may require relocation of the

people/equipment. A process layout (grouping operations by function)

must be replaced by a product layout (arranging operations in the order of flow.)

10Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Batch Process For Volpens, Ltd.

ProductionScheduler

RawMaterials Finished

Goods

Inspect

Test, Pack& Ship

WIPWIP

Cap Assy Pen Assy

Peel&Stick

LabelPress

Material FlowInformation Flow

11Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Takt Time for Volpens, Ltd.

Monthly Demand =

No. of days per month = 16

Daily Demand =

Avail. Time/Day =

Takt Time =

68

12Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Manpower requirements for Volpens Ltd.

Pessimistic times for the current activitiesCap Assembly: 25 secondsPen Assembly: 17 seconds for pens,

5 seconds for shells

Label Making: 28 secondsPeel & Stick: 16 secondsTest,Pack,Ship: 15 seconds

13Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Note that activities 3, 4, 5 are not performed for shells. Also, activity 2 is minimal for shells. The only activity done, for all items, is cap assembly.

The time for activity 3 includes an allowance for changing coils; excluding this allowance, a pessimistic time will be 23 seconds. Use this number for your calculations.

Manpower requirements for Volpens Ltd.

14Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Minimum Manpower Required for Volpens, Ltd.

Monthly Demand for Pens = 48

Monthly Demand for Shells = 20

Total Monthly Demand = 68

Percentage Demand for Pens =

Percentage Demand for Shells =

15Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Activity Pens ( %) Shells ( %) Ave.Time/UnitCap AssemblyPen AssemblyLabel MakingPeel & StickTest,Pack,Ship

_____________Total Labor Content per Unit:Takt Time per Unit:Minimum Manpower Required =

Minimum Manpower Required for Volpens, Ltd.

16Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Mixed Model Sequence for Volpens

Model Monthlydemand

%agedemand

# in thesequence

Blue Pen 24 35% 4Red Pen 14 20% 2Yellow Pen 10 15% 2Blue Shell 6 9% 1Red Shell 8 12% 1Yellow Shell 6 9% 1

Smallest possible sequence length = 1/0.09 = 11.11Choose a 11 unit sequence; adjust every two cycles.

Note: A mixed model sequence is used to intentionally vary work content and component requirements.

17Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Mixed Model Sequence For Volpens

Seq. # Model Desc.1 Blue Pen2 Red Pen3 Blue Pen45

Yellow Pen67

Yellow Shell

8 Red Pen910

Yellow Pen

11

Blue Shell

121314151617

Seq. # Model Desc.181920212223242526272829303132

Red Shell3334

Blue Pen

Blue PenRed PenBlue Pen

Red Pen

Red ShellBlue Pen

Red PenYellow Pen

Blue Shell

Yellow ShellBlue PenRed PenBlue Pen

Yellow Pen

Red Shell

Red PenBlue Shell

Blue Pen

Yellow ShellBlue Pen

Red Shell

Blue Pen

Blue Pen

Yellow Pen

18Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Mixed Model Assembly Schedule for Volpens

Family: Pens Week 3 Week 4 Week 5 Week 6 Blue Pen & Case PCB1 7 5 7 5 Red Pen & Case PCR2 4 3 4 3 Yellow Pen&Case PCY3 2 3 2 3 Totals 13 11 13 11

Family: Shells Week 3 Week 4 Week 5 Week 6 Blue Shell SB1 1 2 1 2 Red Shell SR2 2 2 2 2 Yellow Shell SY3 1 2 1 2 Totals 4 6 4 6

19Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Work Center Part Week 2 Week 3 Week 4 Week 5Cap Assembly CAB1 8 7 8 7

CAR2 6 5 6 5CAY3 3 5 3 5

UnitsWorkloadon Operators

Mixed Model Scheduling: Lean Loading For Caps

20Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

P. Description Part # Week 2 Week 3 Week 4 Week 5Blue Cap CB1 8 7 8 7

Units

P. Description Part # Week 2 Week 3 Week 4 Week 5Red Cap CR2 6 5 6 5

Units

P. Description Part # Week 2 Week 3 Week 4 Week 5Yellow Cap CY3 3 5 3 5

Units

Mixed Model Scheduling: Purchase Orders For Caps

21Ardavan Asef-Vaziri 6/4/2009Lean Thinking: 4- Mixed Model Scheduling

Part8:008:158:308:459:009:159:309:4510:00

Implementing Mixed-Model Scheduling: The Load-Leveling (Heijunka) Box